Method and apparatus for providing common time reference in wireless communication system

ABSTRACT

A terminal and method of performing device-to-device (D2D) communication by a terminal in a wireless communication system is provided. The method includes determining if a signal including synchronization information is received via at least one of an uplink and a downlink; and when the signal including the synchronization information is not received, transmitting a synchronization information-containing signal.

PRIORITY

This application claims priority under 35 U.S.C. §119(a) to KoreanPatent Application No. 10-2013-0011883, filed in the Korean IntellectualProperty Office on Feb. 1, 2013, the entire content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a procedure, operatingmethod, and apparatus for autonomously providing a common time referencefor device-to-device (D2D) communication of a wireless communicationterminal.

2. Description of the Related Art

Wireless mobile communication systems in use today are mainly based on awired network, and wireless communication is actually applied to a linkbetween a base station and a terminal. Thus, if the wired network, onwhich a wireless mobile communication system is based, is damaged whenvarious disasters occur, it is impossible to provide normal wirelesscommunication services. In such emergency conditions, device-to-device(D2D) wireless communication may be used as one way to provide backupwireless communication services for the existing wired network-basedwireless communication services.

The 3^(rd) Generation Partnership Project (3GPP), the standards groupfor asynchronous cellular mobile communication, is also studying thesupport for D2D communication while conducting Long Term Evolution (LTE)release 12 standardization. In particular, 3GPP is currently underresearch and development and standardization aimed at D2D communicationbetween LTE terminals, which supports proximity-based applications andservices for terminals at a relatively long distance, as well asemergency backup communication services between terminals.

In order to achieve D2D communication, initially, a process ofrecognizing the existence of other terminals is required, which isreferred to as a discovery process. Each D2D terminal transmits adiscovery signal to notify other terminals of its existence. Further,each D2D terminal receives discovery signals transmitted from otherterminals to recognize the existence of other terminals. In order toefficiently perform such an operation, a specific time interval whereD2D terminals transmit/receive discovery signals must be predefinedunder the same common time reference. Otherwise, D2D terminals cannotknow when neighboring D2D terminals transmit discovery signals, and thushave to always monitor discovery signals, which results in an increasein power consumption of the terminals.

In order to efficiently perform a discovery process for D2Dcommunication, a common time reference has to be provided to D2Dterminals. When a cellular network is normally operated, the common timereference may be acquired from a synchronization signal transmitted froma base station.

However, when a cellular network cannot be normally operated due tonatural disasters and the like, or when a terminal is located out of thecoverage area of the cellular network, there is a problem in that eachD2D terminal cannot acquire common time synchronization from a basestation.

Accordingly, there is a need for a way to provide a common timereference to D2D terminals. There is also a need to consider a methodfor providing a common time reference not by a base station but by a D2Dterminal.

SUMMARY

The present invention has been made to address at least theabove-mentioned problems and disadvantages, and to provide at least theadvantages described below.

Accordingly, an aspect of the present invention is to provide anoperating method, operation procedure, and apparatus required to supporta function of D2D terminal of providing a common time reference.

According to an aspect of the present invention, a method of performingD2D communication by a terminal in a wireless communication system isprovided. The method includes determining if a signal includingsynchronization information is received via at least one of an uplinkand a downlink; and when the signal including the synchronizationinformation is not received, transmitting a synchronizationinformation-containing signal.

According to another aspect of the present invention, a terminal forperforming D2D communication in a wireless communication system isprovided. The terminal includes a transceiver that transmits andreceives signals to and from other terminals and a base station; and acontroller that controls the transceiver, determines if a signalincluding synchronization information is received via at least one of anuplink and a downlink, and transmits a synchronizationinformation-containing signal when the signal including thesynchronization information is not received.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features, and advantages of certainembodiments of the present invention will be more apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates an example of a resource through which a base stationtransmits a synchronization signal via a downlink and a terminaltransmits a D2D synchronization signal via a downlink;

FIG. 2 illustrates an example of a resource through which a terminaltransmits a D2D synchronization signal via an uplink;

FIG. 3 illustrates another example of a resource through which aterminal transmits a D2D synchronization signal via an uplink;

FIG. 4 is a flowchart illustrating an example of a procedure in which aD2D terminal autonomously provides a common time reference in a wirelesscommunication system according to the present invention;

FIG. 5 is a flowchart illustrating another example of a procedure inwhich a D2D terminal autonomously provides a common time reference in awireless communication system according to the present invention;

FIG. 6 is a flowchart illustrating an example of a procedure in which aD2D terminal autonomously terminates provision of a common timereference in a wireless communication system according to the presentinvention;

FIG. 7 is a flowchart illustrating an example of a procedure performedwhen a D2D terminal, which has been provided with a common timereference, loses the common time reference in a wireless communicationsystem according to the present invention;

FIG. 8 is a flowchart illustrating an example of a procedure in which aD2D terminal provides a common time reference, based on signaling mostrecently received from a base station, which indicates whether it ispossible for the terminal to provide a common time reference, in awireless communication system according to the present invention;

FIG. 9 is a block diagram illustrating a structure of a D2D terminaltransmitter in a wireless communication system according to the presentinvention; and

FIG. 10 is a block diagram illustrating a structure of a D2D terminalreceiver in a wireless communication system according to the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION

Hereinafter, various embodiments of the present invention will bedescribed with reference to the accompanying drawings. Further, in thefollowing description of the present invention, a detailed descriptionof known functions and configurations incorporated herein will beomitted when it may make the subject matter of the present inventionunclear. Terms described below are defined in consideration of functionsof the present invention, but may vary according to the intention of auser or operator. Therefore, the definitions need to be determined basedon the overall contents of the present specification.

In the following embodiments of the present invention, a descriptionwill be given of a procedure and operating method for autonomouslyproviding a common time reference for D2D communication by a terminalcapable of supporting D2D communication (hereinafter referred to as “D2Dterminal”) in a wireless communication system.

First, as an example of a means for a D2D terminal to provide a commontime reference, a D2D synchronization signal is defined. When a cellularnetwork is not available, the D2D synchronization signal provides thesame function as an existing synchronization signal transmitted from abase station. That is, the D2D synchronization signal allows forsynchronization acquisition for D2D communication between terminals, andtransfers ID-related information indicating by which a common timereference is provided. In embodiments of the present invention, when acellular network is normally operated, D2D terminals may acquire acommon time reference from a base station to which they belong and usethe acquired common time reference. As an example, in the case of an LTEsystem, a terminal acquires time synchronization to a cell to which itbelongs and the cell ID of the cell by detecting a synchronizationsignal including a primary synchronization signal (PSS) and a secondarysynchronization signal (SSS), transmitted from a base station, whenaccessing the base station, and the acquired time synchronization may beused as a common time reference.

A method of operating a D2D terminal according to embodiments of thepresent invention includes a process in which, when the D2D terminalcannot acquire a common time reference from a cellular network within acertain period of time, a certain number of attempts, or the expirationof a predetermined timer and subsequently cannot acquire a common timereference from any other D2D terminal within another certain period oftime, another certain number of attempts, or another expiration of apredetermined timer, it autonomously provides a common time reference.

In embodiments of the present invention, whether a D2D terminal acquiresa common time reference provided from a cellular network may bedetermined by whether the D2D terminal successfully detects asynchronization signal and system information from a base station orwhether the D2D terminal successfully detects a synchronization signal.

In embodiments of the present invention, a means for a D2D terminal toautonomously provide a common time reference is a D2D synchronizationsignal transmitted from the corresponding D2D terminal. Also, the D2Dsynchronization signal is a signal that is distinguished from asynchronization signal transmitted by a base station.

In embodiments of the present invention, whether a D2D terminal acquiresa common time reference from another D2D terminal may be determined bywhether the D2D terminal successfully detects a common time referencethat another D2D terminal transmits.

A method of operating a D2D terminal according to embodiments of thepresent invention includes a process in which, when the D2D terminalcannot acquire either a common time reference from a cellular network ora common time reference from any other D2D terminal within a certainperiod of time, a certain number of attempts, or the expiration of apredetermined timer, it autonomously provides a common time reference.

A method of operating a D2D terminal according to other embodiments ofthe present invention includes a process in which the D2D terminalstarts to provide a common time reference, and then autonomouslyattempts to acquire both a common time reference from a cellular networkand a common time reference from another D2D terminal at certain periodsor after a certain period of time, a certain number of times ofproviding a common time reference, or the expiration of a predeterminedtimer. If a corresponding D2D terminal acquires a common time reference,then it stops providing a common time reference, and follows theacquired common time reference.

In embodiments of the present invention, a D2D terminal, which has mostrecently received a configuration set to always provide a common timereference from a base station, follows a common time reference acquiredfrom the base station or another D2D terminal, as described above, andsimultaneously the D2D terminal itself may also provide a common timereference corresponding to the corresponding common time reference. Thisallows the D2D terminal to serve as a detour path for providing a commontime reference to D2D terminals that cannot acquire the common timereference from the base station because of being located in a shadowzone within the coverage area of the base station.

In embodiments of the present invention, whether to acquire a commontime reference from a cellular network may have priority over whether toacquire common time reference from any other D2D terminal, but this mayvary according to configurations. Further, a cellular network mayinclude transmitting and receiving data through LTE or 3G signalstransmitted and received via a base station.

In terms of autonomously providing a common time reference, D2Dterminals are prioritized by differently setting a predetermined periodof time or a predetermined number of acquisition attempts, requireduntil each D2D terminal may autonomously provide a common timereference, according to various factors such as the types or users ofthe D2D terminals.

Embodiments of the present invention include an operation procedure andmethod for autonomously providing a common time reference by a terminalsupporting a D2D communication function.

When a D2D terminal cannot acquire a common time reference from acellular network under a given condition and subsequently cannot acquirea common time reference from any other D2D terminal under a givencondition, the D2D terminal autonomously provides a common timereference. Alternatively, when a D2D terminal does not succeed inacquiring a common time reference from a cellular network and a commontime reference from any other D2D terminal under a given condition, theD2D terminal autonomously provides a common time reference. A means fora D2D terminal to autonomously provide a common time reference is a D2Dsynchronization signal transmitted from the corresponding D2D terminal,and the D2D synchronization signal is a signal that is distinguishedfrom a synchronization signal transmitted by a base station. The presentinvention includes a method for efficiently transmitting a D2Dsynchronization signal via a downlink and an uplink.

Further, if a given condition is satisfied, a D2D terminal, which isproviding a common time reference, autonomously attempts to acquire acommon time reference from a cellular network or another D2D terminal,and when succeeding in acquiring a common time reference, stopsproviding a common time reference and follows the acquired common timereference.

In the above process, whether to acquire a common time reference from acellular network may have priority over whether to acquire a common timereference from another D2D terminal. Further, in terms of autonomouslyproviding a common time reference, D2D terminals are prioritized bydifferentially setting a condition under which each D2D terminal mayautonomously provide a common time reference.

Embodiments of the present invention provide a method for efficientlyacquiring a common time reference in a situation where terminals cannotreceive cellular network services.

FIG. 1 illustrates an example in which a base station transmits asynchronization signal via a downlink and a D2D terminal also transmitsa D2D synchronization signal via the same downlink in a wirelesscommunication system according to embodiments of the present invention.

The following description will be given on the assumption that thewireless communication system is an LTE system. However, it should beclear that those of ordinary skill in the art may also readily applyembodiments of the present invention to other systems.

Referring to FIG. 1, in an LTE system, a set of multiple symbolsconstitutes a time resource unit called a slot 100. Further, a sub-frame101 existing as a time resource unit larger than a slot 100 includes twoslots 100. Further, data is signaled in units of sub-frames 101.Finally, 10 sub-frames 101 collectively constitute a larger timeresource unit called a radio frame 102. Since an LTE downlink employs anOFDM-based transmission scheme, a frequency resource may includemultiple sub-carriers.

Under the above-mentioned time resource configuration, a base stationtransmits the existing LTE synchronization signal via a downlink insub-frames 0 and 5. The synchronization signal transmitted by a basestation (hereinafter referred to as “BS synchronization signal”) mayinclude two synchronization signals, a primary synchronization signal(PSS) 103 and a second synchronization signal (SSS) 104.

The PSS 103 and the SSS 104 are transmitted over the last two symbols ineach first slot of sub-frames 0 and 5, and in the frequency domain,occupy a certain number of sub-carrier resources that are located in themid-portion of the channel bandwidth 105. Further, each of the PSS 103and the SSS 104 includes sequences of a length corresponding to thecertain number of sub-carriers, and the corresponding sequences mayinclude cell ID-related information. The sequences of the PSS 103 andthe SSS 104 are configured to be distinguishable from each other.

That is, the SSS 104 corresponds to a cell group ID, and the PSS 103corresponds to one of N elements in one cell group. Accordingly, whenthe number of cell groups is M, one of M×N cell IDs may be extractedfrom the PSS 103 and SSS 104.

The PSS 103 is transmitted in sub-frames 0 and 5 in the same manner,which makes it possible to acquire sub-frame timing by using the PSS103. Further, the SSS 104 is configured such that two short sequencesare mapped to even-numbered and odd-numbered sub-carriers, respectively,and mapping of the two short sequences when the SSS 104 is transmittedin sub-frame 0 and mapping of the two short sequences when the SSS 104is transmitted in sub-frame 5 are switched, which makes it possible toacquire radio frame timing by using the SSS 104.

In order to newly introduce a D2D synchronization signal to betransmitted by a D2D terminal via a downlink in a situation where thereis already a BS synchronization signal, as in the above-mentioned LTEsystem, the following method may be applied. The D2D synchronizationsignal includes a primary D2D synchronization signal 106 and a secondaryD2D synchronization signal 107.

These D2D synchronization signals are allocated to a resource other thanthat of a BS synchronization signal. As an example, the D2Dsynchronization signals are transmitted over the last two symbols, forexample, in each second slot of sub-frames 0 and 5. In the frequencydomain, the D2D synchronization signals occupy a certain number ofsub-carrier resources that are located in the mid-portion of the channelbandwidth 105. The certain number of sub-carriers may be set tocorrespond to the minimum channel bandwidth supportable in the system.Therefore, when D2D communication is performed without acquisition ofbase station system information, a D2D synchronization signaltransmission and reception process and then a D2D communication processmay be performed on the assumption of the minimum channel bandwidth.Further, the D2D synchronization signal may include different sequencesfrom those of the BS synchronization signal.

Further, in order to allow D2D terminals to distinguish between the BSsynchronization signal and the D2D synchronization signal when acquiringsynchronization, each of the primary D2D synchronization signal 106 andthe secondary D2D synchronization signal 107 includes sequences that aredistinguishable from those of the PSS 103 and the SSS 104. The sequencesof the primary D2D synchronization signal 106 and the secondary D2Dsynchronization signal 107 are also configured to be distinguishablefrom each other. Further, the primary D2D synchronization signal 106 andthe secondary D2D synchronization signal 107 include ID-relatedinformation to identify which device transmits the D2D synchronizationsignal, the ID-related information including at least one of informationon a group ID and the number of elements in one group.

The primary D2D synchronization signal 106 is transmitted in sub-frames0 and 5 in the same manner, which makes it possible to acquire sub-frametiming by using the primary D2D synchronization signal 106. Further, thesecondary D2D synchronization signal 107 is configured such that twoshort sequences are mapped to even-numbered and odd-numberedsub-carriers, respectively, and mapping of the two short sequences whenthe secondary D2D synchronization signal 107 is transmitted in sub-frame0 and mapping of the two short sequences when the secondary D2Dsynchronization signal 107 is transmitted in sub-frame 5 are switched,which makes it possible to acquire radio frame timing by using thesecondary D2D synchronization signal 107.

In embodiments of the present invention, since the D2D synchronizationsignal is transmitted in the same sub-frames as the BS synchronizationsignal, the influence of introduction of the D2D synchronization signalon data scheduling can be minimized. That is, in sub-frames 0 and 5,scheduling restrictions are already placed on a certain number ofsub-carrier resources, which are located in the mid-portion of thechannel bandwidth 105, in the symbols used to transmit the BSsynchronization signal. In particular, when data scheduling is performedin units of sub-frames, there is also the possibility that a schedulermay be implemented not to allocate data to the frequency regions ofsub-frames 0 and 5, in each of which the BS synchronization signal istransmitted. Accordingly, sub-frames already having schedulingrestrictions are used to transmit the D2D synchronization signal.However, in some embodiments of the present invention, it is alsopossible to locate a D2D synchronization signal in other sub-frames, anda D2D terminal may easily perform a discovery process with other D2Dterminals by acquiring synchronization, based on the D2D synchronizationsignal.

According to one embodiment of the present invention, if othersub-frames where no BS synchronization signal is transmitted are usedfor D2D synchronization signal transmission, then schedulingrestrictions may be additionally placed on the corresponding sub-frames.

FIG. 2 illustrates an example in which a D2D terminal transmits a D2Dsynchronization signal via an uplink in a wireless communication systemaccording to embodiments of the present invention.

The following description will be given on the assumption that thewireless communication system is an LTE system. Time and frequencyresource configurations are the same as those in FIG. 1, and any uplinksynchronization signal in FIG. 2 corresponds to a D2D synchronizationsignal because a BS synchronization signal is transmitted via adownlink.

In embodiments of the present invention, a D2D terminal transmits asounding signal in the last symbol interval of a sub-frame in whichsounding transmission is possible according to a configuration from abase station. The D2D terminal may transmit and receive a D2Dsynchronization signal in an interval corresponding to the soundingsignal.

As shown in FIG. 2, the D2D synchronization signal is transmitted over acertain sub-carrier region, which is located in the mid-portion of thechannel bandwidth 200, in the last symbol of each of sub-frames 0, 1, 5,and 6. The certain sub-carrier region may be set to correspond to theminimum channel bandwidth supportable in the system. Therefore, when D2Dcommunication is performed without acquisition of base station systeminformation, a D2D synchronization signal transmission and receptionprocess and then a D2D communication process may be performed on theassumption of the minimum channel bandwidth.

The reason why the D2D synchronization signal is allocated to the lastsymbol of a transmission sub-frame is to make the most of the symbolalready used for sounding transmission. Since a sounding signalgenerally has a long transmission period, and has an advantage in thattransmission resources can be easily adjusted by a configuration from abase station, a D2D synchronization signal may be introduced whilescheduling complexity and the existing system are less affected byintroduction of the D2D synchronization signal, as compared to a methodusing data or control signal resources.

If another terminal serviced by a base station transmits a soundingsignal within a certain distance from a terminal transmitting a D2Dsynchronization signal, then a collision between the sounding signal andthe D2D synchronization signal may occur. Thus, in order to preventthis, the base station may configure terminals within its radius ofcoverage not to transmit a sounding signal in a sub-frame where a D2Dsynchronization signal is transmitted. Further, in some embodiments ofthe present invention, the base station transmits a configuration whichindicates whether to transmit a D2D synchronization signal or whether totransmit a sounding signal to the terminals, and such a configurationmay be differently determined according to communication conditions.

The D2D synchronization signal transmitted in the last symbol of each ofthe four sub-frames includes a primary D2D synchronization signal 201and a secondary D2D synchronization signal 202. In embodiments of thepresent invention, these two synchronization signals may be separatelytransmitted in sub-carrier regions divided in the frequency domain.

As an example, the primary D2D synchronization signal 201 is mapped toeven-numbered sub-carriers, and secondary D2D synchronization signal 202is mapped to odd-numbered sub-carriers in a frequency region that islocated in the mid-portion of the channel bandwidth 200 and is allocatedfor D2D synchronization signal transmission. The divided sub-carrierregions may have various configurations such aseven-numbered/odd-numbered sub-carriers and upper/lower sub-carriers.

As shown in FIG. 2, the D2D synchronization signal is transmitted fourtimes in one radio frame, and sub-carrier regions to which the primaryD2D synchronization signal 201 and the secondary D2D synchronizationsignal 202 are mapped may vary according to these points of time whenthe D2D synchronization signal is transmitted. As an example,sub-carrier regions to which the two D2D synchronization signals aremapped respectively in sub-frames {0, 1} may be interchanged with thosein sub-frames {5,6}, or sub-carrier regions to which the two D2Dsynchronization signals are mapped respectively in sub-frames {0, 5} maybe interchanged with those in sub-frames {1,6}. Radio frame timing maybe acquired using the mapping variation according to the points of timeof D2D synchronization signal transmission.

Further, the primary D2D synchronization signal 201 includes the sameone sequence, which is transmitted in all of sub-frames {0, 1, 5, 6},and the secondary D2D synchronization signal includes two shortsequences, one of which is transmitted in sub-frames {0, 1} and theother of which is transmitted in sub-frames {5, 6}. The primary D2Dsynchronization signal 201 and the secondary D2D synchronization signal202 are interleaved to constitute each sequence. Using this, it ispossible to acquire sub-frame timing. In terms of the number andpositions of sub-frames for D2D synchronization signal transmission andthe transmission pattern at each point of time of D2D synchronizationsignal transmission, various changes and modifications may be made tothe above-mentioned method.

Compared with FIG. 1, the reason why the number of times of D2Dsynchronization signal transmission in one radio frame is increased from2 to 4 is to guarantee the detection performance of the D2Dsynchronization signal. That is, each of the primary and secondary D2Dsynchronization signals fully use the overall frequency region allocatedfor D2D synchronization signal transmission in FIG. 1, but the two D2Dsynchronization signals co-exist in the same frequency region in FIG. 2,and thus the size of the frequency region used by each of the primaryand secondary D2D synchronization signals is reduced by half, ascompared to FIG. 1. Accordingly, in order to compensate for this, thenumber of times of transmission is increased two-fold. However, in someembodiments of the present invention, the number of times of the D2Dsynchronization signal transmission may be changed.

FIG. 3 illustrates another example in which a D2D terminal transmits aD2D synchronization signal via an uplink in a wireless communicationsystem according to embodiments of the present invention.

Referring to FIG. 3, the following description will be given on theassumption that the wireless communication system is an LTE TDD (TimeDivision Duplex) system. However, it will be apparent to those skilledin the art that the method according to embodiments of the presentinvention as described below in FIG. 3 may be applied to othercommunication systems.

Time and frequency resource configurations are the same as those in FIG.1, and any uplink synchronization signal in FIG. 3 corresponds to a D2Dsynchronization signal because a BS synchronization signal istransmitted via a downlink. In an LTE TDD system, sub-frames in oneradio frame are divided into uplink sub-frames and downlink sub-frames,and the uplink and downlink sub-frames are separately used. Inparticular, a plurality of downlink/uplink configurations are predefinedaccording to the numbers and positions of downlink/uplink sub-frames ina radio frame, and a base station selects one of them and uses theselected downlink/uplink configuration.

As shown in FIG. 3, the D2D synchronization signal is transmitted over acertain sub-carrier region, which is located in the mid-portion of thechannel bandwidth 300, in the last two symbol intervals of each ofsub-frames 1 and 6. The certain sub-carrier region may be set tocorrespond to the minimum channel bandwidth supportable in the system,and may be changed in position according to embodiments of the presentinvention. Therefore, when D2D communication is performed withoutacquisition of base station system information, a D2D synchronizationsignal transmission and reception process and then a D2D communicationprocess may be performed on the assumption of the minimum channelbandwidth. In particular, the primary and secondary D2D synchronizationsignals are transmitted in different symbol intervals. In the scenarioof FIG. 3, the reason why the D2D synchronization signal is transmittedin sub-frames 1 and 6 is as follows.

Since sub-frames 1 and 6 are sub-frames that may be configured asspecial sub-frames for switching between downlink and uplink sub-frames,and the last few symbols of each of such sub-frames may be used totransmit a sounding signal, it is intended to use these symbols for D2Dsynchronization signal transmission. If another terminal serviced by theLTE TDD system transmits a sounding signal within a certain distancefrom a terminal transmitting a D2D synchronization signal, then acollision between the sounding signal and the D2D synchronization signalmay occur. Thus, in order to prevent this, the base station mayconfigure terminals within its radius of coverage not to transmit asounding signal in a sub-frame where a D2D synchronization signal istransmitted. In some embodiments of the present invention, the basestation transmits a configuration which indicates whether to transmit aD2D synchronization signal, the transmission timing of the D2Dsynchronization signal, and whether to transmit a sounding signal, tothe terminals. Further, such a configuration may be variably determinedcorresponding to communication conditions.

In embodiments of the present invention, for all the downlink/uplinkconfigurations of the LTE TDD system, sub-frame 1 may be used forsounding signal transmission, and a terminal may transmit a soundingsignal in the last one symbol or two symbols of sub-frame 1, dependingon the configuration received from the base station. However, in someembodiments of the present invention, a sounding signal may betransmitted in other sub-frames.

The example shown in FIG. 3 assumes that the configuration received fromthe base station indicates that a sounding signal may be transmitted inthe last two symbol intervals, and the corresponding symbol intervalsare used for D2D synchronization signal transmission. To the contrary,according to the downlink/uplink configuration, sub-frame 6 is asub-frame where uplink sounding signal transmission is possible, or isused as a downlink sub-frame. Accordingly, in order to smoothly performD2D synchronization signal transmission in sub-frame 6, the base stationneeds to use a downlink/uplink configuration in which sub-frame 6 is notused as a downlink sub-frame. In embodiments of the present invention, aD2D terminal may transmit the D2D synchronization signal in sub-frame 1and sub-frame 6.

FIG. 4 is a flowchart which illustrates an example of a procedure inwhich a D2D terminal autonomously provides a common time reference in awireless communication system according to the present invention.

In this example, it is assumed that the terminal starts to acquireinitial synchronization in a situation where a cellular network cannotprovide services to the terminal.

Referring to FIG. 4, in step 400, the terminal attempts to acquire acommon time reference from a base station, and determines if a commontime reference is acquired under a given condition. Here, the givencondition means that at least one of the elapse of a certain period oftime, the arrival of a certain number of attempts to acquire a commontime reference, and the expiration of a predetermined timer issatisfied. Further, by differently setting the given condition accordingto the types of terminals and users, terminals may be prioritized interms of providing a common time reference. In embodiments of thepresent invention, different priorities may be given to users, and aterminal with a higher priority may preferentially acquiresynchronization.

The priorities of terminals may be set as unique values when theterminals are manufactured, or may be set by a configuration receivedfrom a base station. For example, for users who are responsible toresolve an emergency situation, such as firemen, police, and militarypersonnel, the given condition may be set to a shorter period of time,and thereby higher priorities may be given to them than general users.When a common time reference is first provided from a terminal with ahigher priority, terminals with lower priorities do not autonomouslyprovide a common time reference, and acquire the common time referencefrom the terminal with a higher priority and applies the acquired commontime reference. Accordingly, in this way, other terminals adjacent tothe terminal with a higher priority may acquire synchronization, basedon the common time reference provided from the terminal with a higherpriority.

Further, acquiring a common time reference means that a synchronizationsignal and system information from a base station are successfullydetected or a synchronization signal from a base station is successfullydetected.

If it is determined in step 400 that a common time reference is notacquired under the given condition, then the terminal proceeds to step401, and attempts to acquire a common time reference, based on a signalreceived from another D2D terminal, and determines if a common timereference is acquired under a given condition. Here, the given conditionis the same as described in step 400. However, the length of a period oftime, the value of a number of acquisition attempts, or the expirationtime of a timer may be set differently from that in step 400. Further,acquisition of a common time reference means that a D2D synchronizationsignal from another D2D terminal is successfully detected.

If it is determined in step 401 that a common time reference is notacquired from any other D2D terminal, then the terminal proceeds to step402, and the terminal itself provides a common time reference. In someembodiments of the present invention, a method of transmitting thecommon time reference provided by the terminal itself may includetransmitting a synchronization signal in time and frequency intervalsthat the terminal determines in either a random or predetermined manner.

Subsequently, the terminal ends the algorithm according to the presentinvention.

If it is determined in step 400 that the terminal acquires a common timereference from the base station under the given condition, or it isdetermined in step 401 that the terminal acquires a common timereference from another D2D terminal under the given condition, then theterminal proceeds to step 403, and may acquire synchronizationcorresponding to the common time reference acquired from the basestation or another D2D terminal. Subsequently, the terminal ends thealgorithm according to the present invention.

In some embodiments of the present invention, a discovery process may beperformed in the next step.

FIG. 5 is a flowchart which illustrates another example of a procedurein which a D2D terminal autonomously provides a common time reference ina wireless communication system according to the present invention.

In this example, it is assumed that the terminal starts acquiringinitial synchronization in a situation where a cellular network cannotprovide services to the terminal. However, even when the cellularnetwork can provide services to the terminal, the procedure of FIG. 5may be implemented.

Referring to FIG. 5, in step 500, the terminal attempts to acquire acommon time reference from either a base station or another D2Dterminal, and determines if a common time reference is acquired under agiven condition. In all embodiments of the present invention, theterminal may attempt to acquire a common time reference from at leastone of a base station and another D2D terminal.

Here, the given condition means that at least one of the elapse of acertain period of time, the arrival of a certain number of attempts toacquire a common time reference, and the expiration of a predeterminedtimer is satisfied. However, the length of a period of time, the valueof a number of acquisition attempts, or the expiration time of a timermay be set differently from that in the example of FIG. 4. Further,acquiring a common time reference means that a synchronization signaland system information from a base station are successfully detected, asynchronization signal from a base station is successfully detected, ora D2D synchronization signal from another D2D terminal is successfullydetected.

If it is determined in step 500 that a common time reference is notsuccessfully acquired from either the base station or another D2Dterminal, then the terminal proceeds to step 501, and the terminalitself provides a common time reference. Subsequently, the terminal endsthe algorithm according to the present invention.

If it is determined in step 500 that a common time reference issuccessfully acquired from the base station or another D2D terminal,then the terminal proceeds to step 502, and acquires synchronizationcorresponding to the acquired common time reference. When the terminalacquires a common time reference from both the base station and anotherD2D terminal, it may preferentially follow the common time referencefrom the base station. Subsequently, the terminal ends the algorithmaccording to the present invention.

In some embodiments of the present invention, a discovery process may beperformed in the next step.

FIG. 6 is a flowchart which illustrates an example of a procedure inwhich a D2D terminal autonomously terminates provision of a common timereference in a wireless communication system according to the presentinvention.

Referring to FIG. 6, whether the terminal provides a common timereference is determined in step 600, and the terminal proceeds to step601 when the terminal provides a common time reference. In step 601, theterminal determines if a condition under which to attempt to acquire acommon time reference from a base station or another D2d terminal issatisfied. Here, the condition under which to attempt to acquire acommon time reference means that at least one of the elapse of a certaincyclic period of time during which to attempt to acquire a common timereference, the elapse of a certain period of time starting from a pointof time of provision of a common time reference, and the expiration of apredetermined timer is satisfied.

If it is determined in step 601 that the condition under which toattempt to acquire a common time reference is satisfied, then theterminal proceeds to step 602, and attempts to acquire a common timereference from the base station or another D2D terminal. Subsequently,in step 603, the terminal determines if a common time reference isacquired from the base station or another D2D terminal under a givencondition. Here, the given condition means that at least one of apredetermined period, the arrival of a predetermined period, the elapseof a certain period of time, the arrival of a certain number of attemptsto acquire a common time reference, and the expiration of apredetermined timer is satisfied. However, the length of a period oftime, the value of a number of acquisition attempts, or the expirationtime of a timer may be set differently from those in FIGS. 4 and 5.

If a common time reference is acquired, the terminal proceeds to step604, and the terminal stops providing the common time reference andacquires synchronization, based on the acquired common time reference.When the terminal acquires a common time reference from both the basestation and another D2D terminal, it may preferentially follow thecommon time reference from the base station. Subsequently, the terminalends the algorithm according to the present invention.

Further, when the terminal acquires a common time reference from atleast one of the base station and another D2D terminal, the terminal maytransmit a signal including common time reference information accordingto the acquired common time reference to another terminal or the basestation. In some embodiments of the present invention, signals includingcommon time reference information, which are transmitted when theterminal acquires a common time reference from the base station, whenthe terminal acquires a common time reference from another terminal, andwhen the terminal itself provides a common time reference, may betransmitted in different manners. More specially, these signals may bedifferentiated by being transmitted through the signal transmissionmethods described in FIGS. 1 to 3.

In embodiments of the present invention, when a terminal acquires aplurality of common time references, it may preferentially employ commontime reference information included in a signal transmitted based on acommon time reference acquired from a base station. Further, a terminalmay preferentially employ common time reference information included ina signal transmitted based on a common time reference acquired fromanother terminal, rather than common time reference information acquiredbased on a common time reference provided by the terminal itself.

If it is determined in step 600 that the terminal does not provide acommon time reference, the terminal proceeds to step 605, and follows acommon time reference acquired from the base station or another D2Dterminal.

Further, if it is determined in step 601 that the condition under whichto attempt to acquire a common time reference is not satisfied or it isdetermined in step 603 that a common time reference is not acquired fromthe base station or another D2D terminal under the given condition, thenthe terminal may continue to provide the common time reference.

FIG. 7 is a flowchart which illustrates an example of a procedureperformed when a D2D terminal, which has been provided with a commontime reference, loses the common time reference in a wirelesscommunication system according to the present invention.

Referring to FIG. 7, in step 700, the terminal determines if the commontime reference provided thereto is lost. Here, the loss of a common timereference includes a situation where it is impossible to acquire acommon time reference for a certain period of time or when apredetermined timer expires. If the common time reference is lost, thenthe terminal proceeds to step 701, and attempts to acquire a common timereference from a base station or another D2D terminal.

Subsequently, in step 702, the terminal determines if a common timereference is acquired from the base station or another D2D terminalunder a given condition, and when a common time reference is notacquired, the terminal provides a common time reference in step 703.Here, the given condition means that at least one of a predeterminedperiod, the arrival of a predetermined period, the elapse of a certainperiod of time, the arrival of a certain number of attempts to acquire acommon time reference, and the expiration of a predetermined timer issatisfied. However, the length of a period of time, the value of anumber of acquisition attempts, or the expiration time of a timer may beset differently from those in FIGS. 4 to 6. Subsequently, the terminalends the algorithm according to the present invention.

If the common time reference is not lost in step 700, then the terminalproceeds to step 704, and maintains synchronization corresponding to thegiven common time reference. If it is determined in step 702 that acommon time reference is acquired from the base station or another D2Dterminal, then the terminal proceeds to step 704, and maintainssynchronization corresponding to the acquired common time reference.When the terminal acquires a common time reference from both the basestation and another D2D terminal, it may preferentially follow thecommon time reference from the base station. Subsequently, the terminalends the algorithm according to the present invention.

FIG. 8 is a flowchart which illustrates an example of a procedure inwhich a D2D terminal provides a common time reference, based onsignaling most recently received from a base station, which indicateswhether it is possible for the terminal to provide a common timereference, in a wireless communication system according to the presentinvention. In this example, it is assumed that the terminal startsacquiring initial synchronization in a situation where a cellularnetwork cannot provide services to the terminal. However, the exampledescribed in FIG. 8 may also be applied in the same manner when theterminal acquires a common time reference from the base station as inanother example, and receives a configuration from the base station,which indicates whether it is possible for the terminal to provide acommon time reference. Further, even when the terminal does not receivea configuration indicating whether it is possible for the terminal toprovide a common time reference, it may determine whether to provide theacquired common time reference, based on a predetermined condition.

Referring to FIG. 8, in step 800, the terminal determines if a commontime reference is acquired from the base station or another D2D terminalunder a given condition. Here, the given condition means that at leastone of a predetermined period, the arrival of a predetermined period,the elapse of a certain period of time, the arrival of a certain numberof attempts to acquire a common time reference, and the expiration of apredetermined timer is satisfied.

When a common time reference is not successfully acquired, the terminalproceeds to step 801, and determines if a configuration most recentlyreceived from the base station is set to be able to provide a commontime reference. Such a configuration may be received from another D2Dterminal. Further, in some embodiments of the present invention, such aconfiguration may be included in the form of an indicator in a signalreceived from the base station or another D2D terminal, and the terminalmay provide a common time reference, based on the indicator.

If the configuration received from the base station is set to be able toprovide a common time reference, then the terminal proceeds to step 802,and the terminal provides a common time reference. Subsequently, theterminal ends the algorithm according to the present invention. Uponreceiving the most recent configuration set to be able to provide acommon time reference, the terminal provides a common time reference,and then even when acquiring a common time reference from the basestation or another D2D terminal, may also continue to provide a commontime reference corresponding to the acquired configuration, and theterminal itself may continue to provide a common time reference so longas there is no change in the configuration. In some embodiments of thepresent invention, a case may be assumed in which the terminal acquiresa common time reference from the base station or another D2D terminalwhile providing a common time reference. In such a case, the terminalmay perform at least one of an operation of adjusting synchronization incorresponding to the newly acquired common time reference and anoperation of providing the newly acquired common time reference to otherD2D terminals.

If it is determined in step 800 that a common time reference is acquiredunder the given condition, then the terminal proceeds to step 803, andfollows the acquired common time reference. When the terminalsuccessfully acquires a common time reference from both the base stationand another D2D terminal, it may preferentially follow the common timereference from the base station. Subsequently, the terminal ends thealgorithm according to the present invention.

If it is determined in step 801 that the most recent configurationreceived from the base station is set to be unable to provide a commontime reference, then the terminal does not provide a common timereference, and returns to step 800 and attempts to acquire a common timereference.

FIG. 9 is a block diagram which illustrates a structure of a D2Dsynchronization signal transmitter of a D2D terminal in a wirelesscommunication system according to the present invention. It is assumedthat the wireless communication system uses an OFDM (OrthogonalFrequency Division Multiplexing)-based transmission technology. However,in some embodiments of the present invention, the terminal may use anyother transmission technology.

Referring to FIG. 9, when the D2D terminal is determined to provide acommon time reference, the controller 900 controls the D2D terminal togenerate a D2D synchronization signal as a means to provide a commontime reference. When the controller 900 instructs the D2Dsynchronization signal generator 901 to generate a D2D synchronizationsignal, the D2D synchronization signal generator 901 generates primaryand secondary D2D synchronization signals constituting the D2Dsynchronization signal.

These two D2D synchronization signals may include different sequences.The generated D2D synchronization signal is input into the D2Dsynchronization signal resource allocator 902, and the D2Dsynchronization signal resource allocator 902 allocates the D2Dsynchronization signal to transmission time and frequency resourceregions, as described in connection with the examples of FIGS. 1 to 3.The controller 900 is involved in the resource region mapping such thatdifferent mapping patterns may be applied according to points of time ofD2D synchronization signal transmission, as in the example of FIG. 2.Upon being mapped to resources, the D2D synchronization signal isgenerated into an OFDM symbol signal by passing through the OFDM-basedsignal generator 903, and then the OFDM symbol signal is transmitted.

FIG. 10 is a block diagram which illustrates a structure of a D2Dsynchronization signal receiver of a D2D terminal in a wirelesscommunication system according to the present invention. Although thewireless communication system is assumed as an LTE system, the terminalmay be operated in other communication systems.

Referring to FIG. 10, a received D2D synchronization signal is inputinto the D2D synchronization signal extractor 1001, and the D2Dsynchronization signal extractor 1001 extracts a D2D synchronizationsignal from time and frequency resource regions, to which the D2Dsynchronization signal is mapped when it is transmitted, under thecontrol of the controller 1000. With regard to this, the controller 1000controls the D2D synchronization signal extractor 1001 to extract theD2D synchronization signal in consideration of mapping patternsdifferently applied according to points of time of D2D synchronizationsignal transmission, as described above. The extracted D2Dsynchronization signal is finally detected by the D2D synchronizationsignal detector 1002. The Rx (receive) terminal may acquiresynchronization and the ID of the D2D synchronization signal provider.

Embodiments of the present invention make it possible for D2D terminalsto autonomously acquire a common time reference through a D2Dsynchronization signal and efficiently perform D2D communication whenthey cannot acquire the common time reference from a cellular network.Further, since it is possible to prioritize D2D terminals in terms ofautonomously providing a common time reference, confusion resulting fromthe situation where many D2D terminals each provide common timereferences can be reduced. Further, a D2D terminal providing a commontime reference can autonomously recognize a cellular network, which isnormally operated, to terminate provision of the common time referenceand acquire a common time reference from the corresponding cellularnetwork.

Further, in embodiments of the present invention, a D2D terminalincludes a transceiver that transmits and receives data to and from atleast one of a base station and another D2D terminal, and a controlmodule that controls the operation of the transceiver and controls theoperation of the D2D terminal, based on data transmitted and receivedvia the transceiver.

Further, in embodiments of the present invention, a base stationincludes a transceiver that transmits and receives data to and from atleast one of a D2D terminal and another base station, and a controlmodule that controls the operation of the transceiver and controls theoperation of the D2D terminal, based on data transmitted and receivedvia the transceiver.

Although specific embodiments have been described in the detaileddescription of the present invention, various changes and modificationsmay be made without departing from the spirit and scope of the presentinvention. Therefore, the scope of the present invention should not bedefined as being limited to the embodiments, but should be defined bythe appended claims and equivalents thereof.

What is claimed is:
 1. A method by a terminal in a wirelesscommunication system, the method comprising: identifying whether firstsynchronization information is acquired on a downlink; identifying, ifthe first synchronization information is not acquired on the downlink,whether the first synchronization information is acquired on an uplink;acquiring, if the first synchronization information is not acquired oneither of the uplink or the downlink, second synchronization informationby the terminal, the second synchronization information being randomlyselected; and transmitting, if the first synchronization information isnot acquired on either of the uplink or the downlink, a signal includingthe second synchronization information.
 2. The method as claimed inclaim 1, wherein identifying whether the first synchronizationinformation is acquired comprises: identifying whether the firstsynchronization information is acquired on the downlink during aninterval satisfying a first condition; and identifying whether the firstsynchronization information is acquired on the uplink during an intervalsatisfying a second condition.
 3. The method as claimed in claim 2,wherein identifying whether the first synchronization information isacquired on the uplink comprises: identifying, if the firstsynchronization information is not acquired on the downlink, whether thefirst synchronization information is acquired on the uplink during theinterval satisfying the second condition.
 4. The method as claimed inclaim 2, wherein at least one of the first condition and the secondcondition comprises: at least one of whether a certain period of timeelapses, whether a certain number of attempts are performed, and whethera predetermined timer expires.
 5. The method as claimed in claim 2,wherein at least one of the first condition and the second condition isdifferently set according to a terminal type, and is set such that aspecific terminal transmits a signal including synchronizationinformation earlier than other terminals.
 6. The method as claimed inclaim 1, further comprising: transmitting, if the first synchronizationinformation is acquired on the uplink, a signal related to a device todevice communication, based on the acquired first synchronizationinformation.
 7. The method as claimed in claim 1, wherein transmittingthe signal comprises: transmitting the signal including the secondsynchronization information if the terminal is permitted to transmit thesignal, according to a predefined configuration.
 8. The method asclaimed in claim 1, further comprising: transmitting, if the firstsynchronization information is acquired on the downlink, a signalrelated to synchronization information based on the acquired firstsynchronization information.
 9. The method as claimed in claim 8,wherein transmitting the signal including the second synchronizationinformation comprises: transmitting the signal including the secondsynchronization information based on directions of links on each ofwhich the second synchronization information is acquired.
 10. The methodas claimed in claim 1, further comprising: transmitting, if the firstsynchronization information is acquired on the uplink and the downlink,a signal related to a device to device communication, based on the firstsynchronization information acquired on the downlink.
 11. A terminal ina wireless communication system, the terminal comprising: a transceiverfor transmitting and receiving a signal; and a controller configured tocontrol the transceiver, identify whether first synchronizationinformation is acquired on a downlink, identify, if the firstsynchronization information is not acquired on the downlink, whether thefirst synchronization information is acquired on an uplink, acquire, ifthe first synchronization information is not acquired on either of theuplink or the downlink, second synchronization information by theterminal, the second synchronization information being randomlyselected, and transmit, if the first synchronization information is notacquired on either of the uplink or the downlink, a signal including thesecond synchronization information.
 12. The terminal as claimed in claim11, wherein the controller is further configured to identify whether thefirst synchronization information is acquired on the downlink during aninterval satisfying a first condition; and identify whether the firstsynchronization information is acquired on the uplink during an intervalsatisfying a second condition.
 13. The terminal as claimed in claim 12,wherein the controller is further configured to identify, if the firstsynchronization information is not acquired on the downlink, whether thefirst synchronization information is acquired on the uplink during theinterval satisfying the second condition.
 14. The terminal as claimed inclaim 12, wherein at least one of the first condition and the secondcondition comprises: at least one of whether a certain period of timeelapses, whether a certain number of attempts are performed, and whethera predetermined timer expires.
 15. The terminal as claimed in claim 12,wherein at least one of the first condition and the second condition isdifferently set according to a terminal type, and is set such that aspecific terminal transmits a signal including synchronizationinformation earlier than other terminals.
 16. The terminal as claimed inclaim 11, wherein the controller is further configured to transmit, ifthe first synchronization information is acquired on the uplink, asignal related to a device to device communication, based on theacquired first synchronization information.
 17. The terminal as claimedin claim 11, wherein the controller is further configured to transmitthe signal including the second synchronization information, if theterminal is permitted to transmit the signal, according to a predefinedconfiguration.
 18. The terminal as claimed in claim 11, wherein thecontroller is further configured to transmit, if the firstsynchronization information is acquired on the downlink, a signalrelated to a synchronization information based on the acquired firstsynchronization information.
 19. The terminal as claimed in claim 18,wherein the controller is further configured to transmit the signalincluding the second synchronization information, based on directions oflinks on each of which the second synchronization information isacquired.
 20. The terminal as claimed in claim 11, wherein thecontroller is further configured to transmit, if the firstsynchronization information is acquired on the uplink and the downlink,a signal related to a device to device communication, based on the firstsynchronization information acquired on the downlink.